OB ASSOCIATION - Loose association of 10s or 100s of massive O and B stars scattered over several 100 light years. These form from a giant molecular cloud and are often surrounded by an HII region. Once the radiation and outflows of these short-lived massive stars have dispersed the gas remaining after star formation, the potential well that holds the association together weakens, and the stars drift apart. They are usually found in the spiral arms of galaxies.
OBLATENESS - Departure of a planet from spherical form because of the centrifugal force of the rotation.
Mathematically, oblateness (ε) is defined as the difference between equatorial radius (c) and polar radius (a) divided by the equatorial radius (a):
OBLIQUITY - Angle by which the spin axis of a planet to the plane of its ecliptic differs from 90°.
OCCAM'S RAZOR - Principle that all other things being equal, the simplest explanation is to be preferred.
OCCULTATION - When a large celestial body hides a small one from view (as when the moon moves in front of a star or Jupiter over one of its moons), the event is usually termed occultation rather than eclipse.
OCTAHEDRITE - Commonest type of iron meteorite, composed mainly of taenite and kamacite and named for the octahedral (eight-sided) shape of the kamacite crystals. When sliced, etched with a weak acid, and polished, they display a characteristic Widmanstätten pattern. Spaces between larger kamacite and taenite plates are often filled by a fine-grained mixture of kamacite and taenite called plessite. Octahedrites are divided into several groups based on the width of their kamacite lamellae, and each subgroup is associated with a particular chemical class of iron meteorites. Gibeon is shown below.
OCTET RULE - Atoms tend to gain or lose electrons in order to attain an electron configuration resembling a noble gas. Since the outermost shell in these cases is comprised of an s orbital and three p orbitals, the maximum number of electrons in the valence shell is eight (the octet). Some common elements which do not follow the octet rule are: H (needs only 2 electrons to complete its shell) and B (only needs six electrons).
OHM - Derived unit of electric resistance, defined as the resistance between two points on a conductor when a constant potential difference of 1 volt produces a current of 1 ampere in a conductor; it thus has units of V/A.
OHMIC MATERIAL - Material which obeys Ohm's Law. Ohmic materials have resistances that do not change over a wide range of voltages and currents. For an ohmic device, a graph of voltage, V, across the device vs. the current, I, through that device will yield a straight line, with constant slope equal to the resistance of the device, R:
OHM’S LAW - Observation (first made by Georg Simon Ohm) that applies to ohmic materials or devices, stating that resistance is independent of voltage and current under regular operating conditions. Ohm's Law is NOT the statement V = IR, as many believe. That statement is instead the definition of resistance. Ohm's Law instead says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.
OLBERS’ PARADOX - If the Universe is infinite, uniform, and unchanging then the entire sky at night would be bright; the farther you looked out into space, the more stars there would be, and thus in any direction in which you looked your line-of-sight would eventually impinge upon a star. The paradox is resolved by the Big Bang theory, which stipulates a non-uniform, dynamic, and (probably) finite Universe.
OLDHAMITE - Mn-Ca sulfide, (Mn,Ca)S, found in minor amounts in highly reduced meteorites (aubrites and all E chondrites).
OLIVINE - Group of silicate minerals with a compositional range generally running from forsterite (Mg2SiO4) to fayalite (Fe2SiO4), although a separate series of monticellite, CaMgSiO4, to kirschteinite, CaFeSiO4, also occurs. Complete solid solution occurs in the two series.
Fe-Mg substitution most common in (Fe,Mg) olivines. There is very little solid solution between two series (few mole %). The monticellite-kirschteinite series has larger unit cell than forsterite-fayalite series, accomodating the larger Ca cation. Fe-Mn substituion forms a complete solid solution between tephroite, Mn2SiO4, and fayalite. Thers is more Mn and Ca substitution in Fe-rich end members than Mg-rich end members. Ni is commonly present in Mg-rich olivine.
The olvine strucutre consists of Isolated (SiO4)4- tetrahedra pointing alternately up and down along rows parallel to c axis. Mg2+ and Fe2+ occur in octahedral coordination (M-cations) in two distinct octahedral sites: a distorted M1 site and a more regular M2 site. Layers occur parallel to (100) with edge-sharing octahedra cross-linked by isolated (SiO4)4- tetrahedra. Mg2+ and Fe2+ appear to occupy the M1 or M2 sites without preference in the Mg-Fe series. In CaMgSiO4 olivine, Ca2+ prefers M2 site with Mg2+ in the M1 site.
Olvine has several polymorphs. The α olivine structure described above is only stable in upper part of Earth's mantle. At ~400 km depth, olivine transforms into ringwoodite (γ spinel structure), which is about 10% denser At still greater depths (~670 km), ringwoodite decomposes into magnesiowüstite, (Mg,Fe)O and (MgVI,FeVI)SiVIO3, which has a perovskite structure (Si in octahedral coordination).
Olivine is common in pallasites, chondrites and some chondrites; is it found in terrestrial mafic and ultra-mafic igneous rocks. Pure forsterite is found in terrestrial rocks formed by metamorphism of dolomitic limestone. Fayalite is less common, but does occur in alkali igneous rocks.
OLIVINE-BRONZITE - Obsolete name previously applied to H chondrites, because they are composed of olivine and bronzite (pyroxene with ~20 wt. % FeSiO3).
OLIVINE-HYPERSTHENE - Obsolete name previously applied to L chondrites, because they are composed of olivine and hypersthene (pyroxene with 22-30 wt. % FeSiO3).
OMEGA (Ω) - The cosmological density parameter. This has the components of mass density, ΩM, and vacuum energy density, ΩL.
OORT CLOUD - Vast spherical swarm ~1012-1013 long-period comets orbiting the Solar System with semimajor axes between 1,000 and 50,000 AU (0.25 pc), typically with low orbital eccentricity. The Oort cloud, while roughly spherical at the largest radius, is wedge-shaped where it merges with the outer planet region in the vicinity of the Kuiper belt of comets. The cloud is divided into different regions of dynamical stability: the Kuiper Belt (35-50 AU; affected by planetary perturbations), a dynamically inert region (50-2000 AU; not affected by gravity of planets or stars), the inner Oort cloud (2,000-15,000 AU; affected by galactic tidal forces), and the outer Oort cloud (15,000-100,000 AU; affected by stellar perturbations). The Oort cloud has never been observed rather its existence is inferred from analysis of the orbits of comets, which come in from the cloud. Recent discoveries of Kuiper Belt objects have confirmed its existence.
OPEN CLUSTER - Cluster of stars usually containing several 100 members packed into a region usually less than 20 light years in size. They are normally found near regions of star formation in the spiral arms of galaxies.
OPPOSITION - An object is at opposition when it is on the opposite side of the Earth from the Sun. The elongation of a Solar System body at opposition is 180°. The inferior planets, and other objects with orbits closer to the Sun than the Earth, can never be at opposition. Searches for new faint Solar System objects, such as Kuiper Belt Objects and asteroids, often attempt to find these objects at opposition when they will have their maximum illumination by the Sun (i.e. their phase, if discernable, will be full).
ORBITAL ELEMENTS - Quantities used to describe exactly where planet is in its orbit and how the orbit is oriented. Elements defining the shape of an orbit are the semi-major axis (a), orbital eccentricity (e) and perihelion distance (p). Orbital elements describing the orientation of an orbit are orbital inclination (i), ascending node (Ω), and argument of perihelion (ω). Orbital elements defining the position and speed of a planet in its orbit are orbital velocity (v), mean anomaly (M), and mean daily motion (d). Finally, it is necessary to specify the epoch (t0), or reference date of the coordinate system; usually given as the time when the planet is at its closest approach to the Sun. All of the above has assumed orbits around the Sun; however, these orbital elements apply equally to the orbits of stars around each other and the orbits of satellites around their parent planet.
ORDINARY CHONDRITE - A chondrite that is composed mostly of olivine, orthopyroxene, and a certain percentage of more or less oxidized Ni-Fe metal. Based on the differing content of metal and mineralogical compositions, the ordinary chondrites are divided into three distinct categories: H Group (high iron), L Group (low iron), and LL Group (very low iron).
Subtypes of ordinary chondrites are determined using the petrographic criteria given in the table below.
|Olivine & Pyroxene Homogeneity||> 5 % mean deviation||homogeneous|
|Feldspar||minor primary in 3; increase from <5 mm in 3 to >50 mm in 6|
|Chondrule & Matrix Glass||present in decreasing abundance in 3; devitrified to absent in 4 to 6|
|Matrix||fine grained, clastic, minor opaque in 3; recrystallized, coarsening in 4 to 6|
|Chondrule Definition & Abundance||sharply defined in 3; become more diffuse and less abundant in 4 to 6|
For very low Type 3 subtypes, division may be made using the average concentration and standard deviation of Cr2O3 contents in olivine (Grossman & Brearley, 2005).
ORGANIC - Pertaining to C-containing compounds. Organic compounds can be formed by both biological and non-biological (abiotic) processes.
ORGUEIL METEORITE - A large carbonaceous chondrite that disintegrated and fell in fragments near the French town of Orgueil on May 14, 1864. About 20 pieces, totaling ~12 kg in mass, were subsequently recovered from an area of several square km, some head-sized but most were smaller than a fist. Specimens could be cut with a knife and, when sharpened, pieces could be used like pencils.
In 2001, researchers found that a pristine piece of the interior of Orgueil contained a relatively simple mixture of amino acids, consisting primarily of glycine and β-alanine. They also analyzed the sample’s carbon isotope concentration and found that the amino acids were not derived from Earthly contamination but instead were almost certainly synthesized chemically in space. The research team then compared their results with three other meteorites: Murchison and Murray, which have been studied extensively, and Ivuna, a meteorite that fell in Tanzania, Africa, in 1938, that had not been analyzed for amino acids. The team broke the meteorites down into two classes.The Murchison and Murray meteorites were placed in a category containing a complex mix of amino acids made up of more than 70 different types of amino acids. Orgueil and Ivuna, however, were categorized with a much simpler composition made up primarily of just two amino acids.
Murchison and Murray are widely believed to be pieces of an asteroid, as are virtually all meteorites scientists have studied. However, Orgueil and Ivuna show evidence that they are derived from a comet. The amino acid signatures within Orgueil and Ivuna suggest that these compounds were likely synthesized from components such as hydrogen cyanide, which have been recently observed in the comets Hale-Bopp and Hyakutake. This suggests that the organic material in Orgueil and Ivuna is the product of reactions that once took place in the nucleus of a comet, which, if true, would make these meteorites the first to be identified as having come from a comet nucleus.
ORIENTED METEORITE - Meteorite with a conical shape, which results when a meteorite has a stable flight through Earth's atmosphere and undergoes uniform ablation. The leading side and sides often have flow-lines in the surface which indicate the direction the material was being ablated. The rear edges of the trailing side can create a "roll-over" rim or "lip" where the ablating/melting material has started to flow over onto the back side.
ORTHOPYROXENE - Orthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 - to Fe-rich ferrosilite, FeSiO3. Enstatite (en) and ferrosilite (fs) form almost complete solid solution (Mg2+ substitutes for Fe2+ up to about 90 mol. %. Calcium substitution is no more than ~5 mol. % (higher Ca2+ contents occur in pigeonite). Ferrosillite is rarely found in nature; under most geological conditions fayalite + quartz is more stable than ferrosillite: Fe2Si2O3 → Fe2SiO4 + SiO2. See Pyroxene entry for for more information.
ORTHOPYROXENITE (OPX) - A rock composed primarily of orthopyroxene. Non-terrestrial orthopyoxenites include diogenites from 4 Vesta and a single martian meteorite ALH 84001, which was found in the Allan Hills region of Antarctica in 1984. ALH 84001 is a cumulate rock consisting of 97% coarse-grained, Mg-rich orthopyroxene, with small amounts of plagioclase, chromite, and carbonate. It was initially classified as a diogenite, however, the presence of oxidized Fe in chromite led to its reclassification as a Martian meteorite. This was subsequently confirmed by determination of its oxygen isotope composition. With a crystallization age of ~4.4 Ga, it is by far the oldest meteorite from Mars yet discovered. It probably represents a sample of the early crust, providing evidence for the earliest geologic history of Mars.
ORTHOSILICATE - Mineral group also called "nesosilicates," containing isolated SiO4 tetrahedra that are bound by interstitial cations. Their structures are usually closely packed resulting in high densities and hardnesses, with fairly uniform properties in all crystallographic directions. Substitution of Al3+ for Si4+ in is generally low. The most common othrosilicate is olivine, which is ubiquitous in terrestrial and extraterrestral rocks. Also common are garnet, aluminosilicates, zircon and titanite (sphene).
OUT-OF-PHASE - Situation with waves when the peaks from one wave are exactly matched by the troughs of a second wave. If these two waves collide and have the same amplitude, they will mutually cancel. That is, for every positive point along one wave, the second wave would have a negative counterpart of the same magnitude. When these points are added together, they cancel leaving no net amplitude. This kind of phenomena is often called destructive interference.
OXIDATION - Chemical reaction in which electrons are formally transferred between atoms in the reactants and products. When iron rusts it is undergoing an oxidation reaction.
OXYGEN (O) - Element that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only ~1/1500 that of H. Atmospheric oxygen normally occurs in molecular form, O2. Ozone, O3, and monatomic oxygen, O2-, predominate in the upper atmosphere, where ozone shields the Earth from the Sun's UV radiation. Oxygen combines with most other elements and, when this reaction occurs rapidly (with release of heat and light), it is described as combustion, or burning. Oxygen usually occurs bonded with another element (e.g., CO, CO2), being too reactive to persist very long as O2. All O2 in Earth's atmosphere is constantly renewed by photosynthetic activity. Oxygen is very common as ionic crystalline solids bonded with various metal ions to yield "oxides" and bonded with Si to form silicates.
Oxygen has three stable isotopes: 16O (99.757%), 17O (0.038 %), and 18O (0.205 %). Variations in the 18O/16O and 17O/16O ratios are reported in terms of their deviations (δ18O and δ17O) in parts per thousand (‰) from a standard. The usual reference is standard mean ocean water (SMOW), which plots at (0,0). Pure 16O plots at -1000 ‰ on both axes.
The oxygen atoms in terrestrial (and lunar) rocks do not have identical proportions of the three isotopes, but the ratios of these isotopes follow a simple relationship controlled by their masses. Rocks with the same 18O/16O ratio will have the same 17O/16O ratios. But if their 18O/16O ratios differ by say 0.2 %, their 17O/16O ratios will differ by half this amount, 0.1 %. This behavior, called "mass-dependent fractionation," yields the "terrestrial mass-fractionation line" (slope = 0.52) on the isotope plot. All terrestrial rocks plot along this line, as do materials from the Moon. This coincidence of Earth and Moon isotopes is taken as evidence that the two bodies share a common origin. The mass-fractionation lines for other planets (Mars, 4 Vesta) are parallel, indicating slightly different ratios of the three isotopes, but still mass-dependent fractionation. In contrast, meteorites show evidence of formation in many isotopically distinct environments, permitting classification based on oxygen isotopic compositions.
OXYGEN FUGACITY - Measure of the availability of oxygen to react.